In the design and development of a product, files of deliverables such as a specification document, a design document, and verification data generated in design and development processes are managed. Examples of such products include a control system and an embedded system, etc. used in the automotive industry, the electrical industry, and the precision industry, etc. When file management is performed in this way, interrelated files are associated with each other so that in case of any change in part of specification or design, the extent of influence of the change can be found. This file association is aimed at improving traceability.    Patent document 1: Japanese Laid-open Patent Publication No. 09-223008    Patent document 2: Japanese Laid-open Patent Publication No. 10-254691    Patent document 3: Japanese Laid-open Patent Publication No. 09-101884    Patent document 4: Japanese Laid-open Patent Publication No. 08-194610
However, as will be explained below, the above-described search for the extent of influence has a problem that it is naturally limited in narrowing the extent of influence. Namely, in the above-described search for the extent of influence, a series of associations are retrieved in all directions from a file to be changed. As a result, in the search for the extent of influence, a largely-unaffected file is vainly searched. In this manner, in the above-described search for the extent of influence, less-useful files are considerably contained in a search result, and therefore, the extent of influence identified as the search result is expanded.